The most ambitious idea ever outlined by scientists has suffered a remarkable setback. It has been dismissed as a theoretical cul-de-sac that has wasted the academic lives of hundreds of the world's cleverest men and women.

This startling accusation has been made by frustrated physicists, including several Nobel prize winners, who say that string theory - which seeks to outline the entire structure of the universe in a few brief equations - is an intellectual dead end.

Two new books published in America question its very basis. Far from providing mankind with the answers to the mystery of the cosmos, the theory is bogus, they claim.

As one scientist put it: 'The uncritical promotion of string theory is now damaging science.'

However, string theory proponents - who also include several Nobel prize winners - have denounced the criticisms and robustly defended their field. It has already led to many major breakthroughs in mathematics and physics, they say.

Suddenly string theory is tying scientists in knots - although the idea's origins are innocuous enough, and can be traced to physicists' attempts to get out of an intellectual impasse.

Last century, they created quantum mechanics to explain how tiny things - atoms and electrons - behave, while Einstein produced his theory of general relativity to account for the behaviour of huge objects such as galaxies.

Both theories work well - but they are incompatible. Quantum physics cannot explain massive things and relativity cannot account for little ones. By comparison, biologists have Darwin's theory of natural selection to explain living things, big and small, from whales to bacteria. Physicists have no unified code - a prospect that upset Einstein so much that he spent his last 20 years hunting, fruitlessly, for a unified theory of everything.

Then, in the Eighties, a group of scientists created string theory. Matter is not made up of small dot-like entities such as neutrons or quarks, they claimed, but of incredibly small threads of energy that vibrate. A string that vibrates one way becomes an electron. Another, vibrating differently, becomes a neutron. And another becomes one of the carriers of the force of gravity.

'You can think of the universe as a symphony or a song - for both are made up of notes produced by strings vibrating in particular ways,' said Professor Michael Green of Cambridge University.

It sounds intriguing. Unfortunately, to make their equations work, scientists had to add another six dimensions to the universe: four were not enough, though we cannot see these extra dimensions because they are so tightly crumpled up that they are invisible, it was argued. To the general public, of course, all this is faintly baffling.

Nevertheless, string theory proved encouragingly effective - at a theoretical level - to explain both the very small and the incredibly large, and so it began to dominate the study of fundamental physics at universities through the world. According to protagonists, it would soon be possible to describe the cosmos in a few simple equations that could fit on a T-shirt.

But as the years have passed, scientists failed to produced a single practical observation to support the theory. One problem, they said, was that the energy needed to break open matter and study the strings inside it is so colossal that it would require machines big enough to cover the planet.

On top of these problems, recent calculations have produced a surprising prediction from string theory: that there may be an almost infinite number of different universes, some of which would be like our own, and others that would be very different.

And it is at this point that the rot set in. An unprovable theory that talks of unseeable parallel universes and 10-dimensional space has proved too much for some physicists. 'Quasi-theology' and 'post-modern' have been among the most polite terms used; 'bogus' and 'nonsense' among the less forgiving.

'Far from a wonderful technological hope for a greater tomorrow, string theory is the tragic consequence of an obsolete belief system,' said Stanford University's Robert Laughlin, winner of the 1998 Nobel prize for physics.

For a theory that purports to explain the entire structure of the universe, such a high-level attack is very serious. Nor is Laughlin alone: for example, Peter Woit, of Columbia University, and Lee Smolin, of the Perimeter Institute, Canada, have just published books attacking string theory.

'Too many people have been overselling very speculative ideas,' said Woit - author of Not Even Wrong - last week. 'String theory has produced nothing.'

This point was backed by Smolin, whose book is called The Trouble with Physics. Scientists have poured all their energies into a theoretical approach that is proving sterile, he said. 'It is as if every medical researcher in the world had decided there was only way to fight cancer and had concentrated on this line of attack, at the expense of all other avenues,' he said. 'Then that approach is found not to work and scientists discover they have wasted 20 years. That's the parallel with string theory.'

Part of the problem, say critics, is that, in the Eighties, talented young physicists were encouraged by professors to take up string theory because of its immense promise. Now they are middle-aged department heads who have committed their lives to the subject and cannot see it is bogus. It is the scientific equivalent of the emperor's new clothes.

Not surprisingly, such accusations are angrily rejected by string theorists. A theory of everything cannot be created overnight, they argue. It is like complaining about the sound made by an unfinished violin. 'String theory is on the right path,' said David Gross, of the University of California, Santa Barbara, and another Nobel prize winner. 'But this path is quite long. Further breakthroughs are required.'

Nor is it correct to argue that the theory is wrong because it makes no provable or disprovable predictions, said Sanjaye Ramgoolam, of Queen Mary, University of London. 'There are a number of ways that we could prove - or disprove - string theory. For example, Europe's new Large Hadron Collider [being built at Cern in Geneva] may well be powerful enough to provide evidence that suggests we are on the right road.'

And as for the notion that string theorists have their heads stuck in the sand and refuse to see the truth, this is firmly rejected by Green: 'All scientists are excited by new ideas. That is why we are scientists. But when it comes to a unified theory, there have been no new ideas. There is no alternative to string theory. It is the only show in town - and the universe.'

A dinner party guide to string theory

· Matter is made up of infinitesimally small strings of vibrating energy.

· Different vibrations produce different particles, like the quark and the electron.

· We live in a 10-dimensional universe.

· Proponents say it is the only hope we have of producing a unified theory of everything, the holy grail that eluded Einstein.